Gas diffusion electrodes containing modified carbon products

ABSTRACT

Gas-diffusion electrodes containing modified carbon products are described wherein the modified carbon product is a carbon product having attached at least one organic group. The modified carbon product can be used for at least one component of the electrodes such as the active layer and/or the blocking layer. Methods to extend the service life of electrodes as well as methods to reduce the amount of fluorine containing compounds are also described.

This application is a divisional of U.S. patent application Ser. No.09/860,952 filed May 18, 2001, now U.S. Pat. No. 6,630,268 which is acontinuation of prior U.S. patent application Ser. No. 09/415,741 filedOct. 12, 1999, now U.S. Pat. No. 6,280,871 B1.

BACKGROUND OF THE INVENTION

The present invention relates to electrodes and the use of gas-diffusionelectrodes in a variety of applications. The present invention furtherrelates to methods of preparing gas-diffusion electrodes, including thecarbon supports for gas diffusion electrodes. The present invention alsorelates to materials particularly suitable in the manufacture ofimproved gas-diffusion electrodes, such as air diffusion electrodes.

With respect to gas-diffusion electrode structures, multi-layeredcomposite electrodes are the preferred solution. Depending on the finaluse of the electrode, two or more layers of carbon blacks combined withfluorine containing compounds are joined into a continuous structure.Double-layered electrodes have a highly hydrophobic carbon support layercoupled to a lesser hydrophobic layer (also known as the active layer)containing catalyzed carbon and suitable binders. A metal screenembedded in the carbon serves as the current collector. The hydrophobicpart of the electrode contains a gas feed channel so that the reactantgas can easily diffuse through the pores towards the electroactivelayer, where reactions take place. This part of the electrode acts as abarrier to prevent penetration of the electrolyte. Electrolyte in thepores would prevent the diffusion of gas to the reaction layer and thiswould result in a dramatic deterioration of the electrode's performance.As stated before, the active layer is less hydrophobic than thediffusion layer to ensure partial wetting of the carbon and theelectrocatalyst particles. In the active layer, which is also known asthe catalyst layer, the reactant gas supplied from the blocking layerdiffuses in the gas channels to be dissolved in an electrolyte incontact with carbon or catalyzed carbon so that the electrode reactionis carried out on the carbon or catalyzed carbon in the electrolyte. Thestructure and the hydrophobic properties of the active layer can beimportant for efficient electrode operation. It is generally recognisedthat the major concern in developing an efficient electrode is toimprove the wettability of the active layer.

In a three-phase reaction system, such as the active layer, a stableinterface between the electrolyte and the gas has to be maintained sothat the number of reaction sites remains as high as possible for longoperation times. Regarding this point, the ratio of liquid and gas poresin the active layer determines the mass transfer conditions. Poorlywetted pores will result in an acceptably high electrical resistivityand will have low catalyst utilization due to lack of electrolyte,whereas a more hydrophilic interface may flood. Pores with an optimalwettability are filled with only a small film of electrolyte so that thegas diffusion limitations are significantly reduced. The electrolytequantity in the active layer can be adjusted by a change in thefluoropolymer content in the active layer.

A great variety of wet proofed gas diffusion electrodes exist at thepresent time which differ in overall structure and configuration. A gasdiffusion electrode is generally produced by mixing conductive carbonfine powder and the hydrophobic/hydrophilic fluorine resin powder orsuspension thereof, forming the mixture into a sheet, and sintering thesheet.

Water-repellent structures of the diffusion layer are generally achievedby coating the surface of some carbon particles with a hydrophobicmaterial. Polytetrafluoroethylene (PTFE) is one of the most stable andeffective hydrophobic agent known. The most popular of the PTFEmaterials used is in the form of a colloidal suspension, produced by DuPont de Nemours and Co, Inc. under their Teflon® trademark (Teflon30-N). The incorporation of PTFE in the blocking layer serves twofunctions: binding the high surface carbon particles into a cohesivestructure and imparting hydrophobicity to the layer.

The most common method to make carbon more hydrophobic is a wetapplication method. A colloidal aqueous PTFE suspension is blended withcarbon powder in an alcohol/water solution to give a mixture containing5-60 wt. % Teflon®. This mixture is normally produced in the form of anaqueous paste, and it can be rolled, spread, printed, or sprayed onto asubstrate, for example a carbon paper. For example, U.S. Pat. No.5,531,883, incorporated in its entirety by reference herein, relates amethod for preparing a hydrophobic support layer by dispersing carbonblack in water and adding an aqueous dispersion of PTFE.

U.S. Pat. No. 5,561,000, also incorporated in its entirety by referenceherein, relates to a process in which a mixture of carbon and a PTFEsuspension is filtered and the filtered-off paste is spread out on acarbon sheet which has been previously soaked in a hydrophobic renderingmaterial such as PTFE in a suspension. The filtered-off paste is appliedand pressed in the carbon support by means of a scraping knife. Somecathode structures utilise layers of polytetrafluoroethylene to formprotective or backing sheets in order to further increase thehydrophobicity of the carbon black cathodes on the air side.

In the active layer or catalytic layer, a semi-hydrophobic structure ispreferred for a more efficient use of the catalyst, and consequentlyhydrophilic ingredients are used in the air electrode preparation.

The most common method to make carbon partly hydrophilic consists inpreparing an alcohol mixture of the carbon powder (with or withoutcatalyst) and a hydrophilic fluorinated resin. One of the most popularhydrophilic fluorinated polymers available on the market is a perfluoricsulphonic acid polymer produced by Du Pont de Nemours and Co, Inc. undertheir Nafion® trademark (Nafion solution SE-5112). For example, in U.S.Pat. No. 4,877,694, also incorporated in its entirety by referenceherein, a mixture of finely powdered active material is prepared byblending catalysed carbon particles together with an alcohol solution ofNafion.® The resulting mixture is dried and finely chopped. An alcoholdispersion of the above product is then filtered on a first preparedbacking layer to form a dual phase sheet which is dried and sinteredunder pressure.

Several further techniques have been developed to increase the catalystutilization. According to a number of techniques, the catalyst isapplied directly on a solid electrolyte membrane and not on theelectrode. For example, U.S. Pat. No. 5,561,000 relates to a method formaking a gas diffusion layer wherein a catalytic layer is formed on aporous hydrophobic back support in the form of a liquid ink prepared bymixing catalyst particles (20% Pt/C) and a proton conductive monomersolution, such as 5% solution of Nafion®. A non-catalytic intermediatelayer containing a mixture of an electron conductive material, such ascarbon, and the proton conductive monomer is provided between thesupport and the catalytic layer. In some cathode structures the solutionis made of Pt/C catalyst powder, a Nafion® solution, PTFE in suspension,and carbon black and it is applied directly on a Nafion® membrane.

The problem with these methods is the difficulty in simultaneouslyobtaining the required porosity and firmness of the layers provided onthe support. All the above-mentioned patents relate to the use of carboncombined with a colloidal mix, dry mix, or fluorinated polymersolutions. All of the proposed processes involve coating carbon blackparticles with a fluorinated polymer compound. Although theabove-mentioned literature may provide methods which may provide carbonwith the proper hydrophobicity/hydrophilicity balance, the methodsrequire elaborate and complex steps or require relatively expensive rawmaterials.

SUMMARY OF THE INVENTION

A feature of the present invention is to provide gas diffusionelectrodes in which the formulation of each single layer is made mostsuitable for its specific function.

Another feature of the present invention is to provide gas diffusionelectrodes with a precisely controlled degree of hydrophobic and/orhydrophilic characteristics by using carbon particles modified withfunctional groups.

A further feature of the present invention is to provide a method ofobtaining carbon supports for gas diffusion electrodes which preferablyuses less fluorine containing compounds.

Accordingly, the present invention relates to gas diffusion electrodes.The gas diffusion electrodes contain at least a blocking layer and/or anactive layer. The blocking layer, active layer, or both contain at leastone modified carbon product and at least one binder. The modified carbonproduct is a carbon product having attached at least one organic group.

The present invention further relates to methods to improve the servicelife of a gas diffusion electrode by forming a blocking layer or activelayer or both from at least one modified carbon product and at least onebinder.

In addition, the present invention relates to a method to reduce theamount of fluorine-containing compounds in a gas diffusion electrode byforming a blocking layer, active layer, or both with at least onemodified carbon product and at least one binder.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are intended to provide further explanation of the presentinvention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a comparison of potential-time curves under constant load of200 mA/cm² and 23° C. with Electrode A representing treated VulcanXC-72R carbon-based electrode, Electrode B representing untreated VulcanXC-72R carbon-based electrode; and Electrode C representing untreatedShawninigan carbon-based electrode.

FIG. 2 is an operational curve for the Electrode A under current load of200 mA/cm² and 23° C.; and a performance curve for the Electrode C isalso shown for comparison.

FIG. 3 shows current-potential curves for oxygen reduction in 7.5 M KOHand 23° C. on electrodes of types A and C, respectively.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention relates to gas diffusion electrodes, such as theones used in metal-air batteries, fuel cells, and the like. Inparticular, these electrodes have at least one component which containsa modified carbon product. The modified carbon product is a carbonproduct having attached at least one organic group. The use of amodified carbon product preferably leads to one or more advantages suchas longer service life of the electrode and further can permit adecrease in the amount of polymeric binders used and can even permit adecrease or elimination of fluorinated compounds in the electrode whichcan be quite expensive. Gas-diffusion electrode is a general categorywhich includes air-diffusion electrodes, wherein the air diffusionelectrodes can be used in metal-air batteries and fuel cells, and thelike. The “gas” in the gas diffusion electrode includes, but is notlimited to, air, oxygen, CO₂, H₂, NO₂, and the like.

With respect to the air-diffusion electrode, which is generally used inmetal-air batteries and fuel cells, this type of electrode generally isconstructed to have a blocking layer and an active layer. The presentinvention can also be used in gas diffusion electrodes where an activelayer is only present or a blocking layer is only present.

In the present invention, the blocking layer, the active layer, or bothcontain at least one modified carbon product and at least one binder.The blocking layer in the present invention serves the same purpose andfunction as any other blocking layer in a gas-diffusion electrode.Likewise, the active layer also functions and provides the same purposeas any other active layer in a gas-diffusion electrode.

In more detail, the blocking layer is a layer which separates the airfrom the electrolyte. The blocking layer however will permit the entryor diffusion of air through the blocking layer in order to contact theelectrolyte which enters the active layer in order to promote what isknown in the art as the tri-phase or three-phase system. The blockinglayer or diffusion layer will typically be hydrophobic in nature inorder to prevent the electrolyte which is typically aqueous fromescaping through the electrode. The active layer on the other hand hasboth hydrophobic and hydrophilic qualities because the goal of theactive layer is to permit some electrolyte from entering the activelayer to wet the carbon material which in part forms the active layerbut to also permit air to enter from the diffusion layer to contact theelectrolyte in the active layer. Thus, a hydrophilic/hydrophobic balancein the active layer is preferred in order to optimise the operation ofthe electrode. As mentioned above, if the active layer is overlyhydrophilic, then a flooding of the active layer can occur whereas ifthe active layer is only hydrophobic, then electrolyte will have greatdifficulty in wetting any of the carbon which in part forms the activelayer and thus no interaction will occur between the air and theelectrolyte in the active layer.

In the embodiment of the present invention, the blocking layer, theactive layer, or both contain at least one modified carbon product. Informing the blocking layer or the active layer, typically the modifiedcarbon product will be combined with at least one binder to form a pastewhich will then be used to form a layer. The paste which forms the layeris typically put on a conductive substrate such as a nickel substrate orother conductive metal substrate or material. While the blocking layerand/or the active layer can contain any type of modified carbon product,when a modified carbon product forms the blocking layer, it is preferredthat the modified carbon product be hydrophobic in nature. Thus, it ispreferred that the modified carbon product comprise at least one carbonproduct having attached at least one organic group which is hydrophobicin nature. In other words, it is preferred that a hydrophobic organicgroup be attached to the carbon product to form the modified carbonproduct.

Examples of hydrophobic organic groups which are attached to the carbonproduct include, but are not limited to, 1) saturated and un-saturatedalkyl groups, aryl groups, ethers, poly ethers, 2) fluorinated saturatedand unsaturated alkyl groups, aryl groups, ethers, poly ethers; 3) polyor oligo fluorinated compounds, and the like.

Preferably, the organic group which is attached to the carbon product topromote the hydrophobic properties has the general formula -A-R, whereinA is an aromatic group and/or an alkyl group and R represents fluorineand/or a fluorine containing substitutent. The alkyl group is preferablya C₁-C₂₀ alkyl group and more preferably is a C₁-C₁₂ alkyl group. Thearomatic group can include multiple rings. Also, more than one R groupcan be located on the aromatic group and each of these R groups can bethe same or different. More preferably, the hydrophobic group is Ar—CF₃where —CF₃ is preferably in the meta position.

With respect to the active layer, as stated earlier, preferably theactive layer contains a modified carbon product which promoteshydrophilic and hydrophobic characteristics. In order to promote thehydrophilic characteristics of the carbon product which has a tendencyto be naturally hydrophobic, the carbon product preferably has attachedat least one type of hydrophilic organic group which can be an aromaticor alkyl group substituted with an ionic group, an ionizable group; anon-ionic hydrophilic group; or a mixture thereof. Preferably, thehydrophilic type organic group is a sulfophenyl group or a carboxyphenylgroup, or salts thereof. Examples of the ionic or ionizable groupinclude, but are not limited to, sulfonilic acid groups and saltsthereof. Alternatively, the carbon product can have attached at leastone type of hydrophobic organic group and can be used in forming theactive layer.

In more detail, ionizable functional groups forming anions include, forexample, acidic groups or salts of acidic groups. The organic groups,therefore, include groups derived from organic acids. Preferably, whenthe organic group contains an ionizable group forming an anion, such anorganic group has a) an aromatic group or a C₁-C₁₂ alkyl group and b) atleast one acidic group having a pKa of less than 11, or at least onesalt of an acidic group having a pKa of less than 11, or a mixture of atleast one acidic group having a pKa of less than 11 and at least onesalt of an acidic group having a pKa of less than 11. The pKa of theacidic group refers to the pKa of the organic group as a whole, not justthe acidic substituent. More preferably, the pKa is less than 10 andmost preferably less than 9. Preferably, the aromatic group or the alkylgroup of the organic group is directly attached to the carbon product.The aromatic group may be further substituted or unsubstituted, forexample, with alkyl groups. The C₁-C₁₂ alkyl group may be branched orunbranched and is preferably ethyl. More preferably, the organic groupis a phenyl or a naphthyl group and the acidic group is a sulfonic acidgroup, a sulfinic acid group, a phosphoric acid group, or a carboxylicacid group. Examples include —COOH, —SO₃H and —PO₃H₂, —SO₂NH₂,—SO₂NHCOR, and their salts, for example —COONa, —COOK, —COO⁻NR₄ ⁺,—SO₃Na, —HPO₃Na, —SO₃ ⁻NR₄ ⁺, and PO₃Na₂, where R is an alkyl or phenylgroup. Particularly preferred ionizable substituents are —COOH and —SO₃Hand their sodium, potassium, lithium salts. It is understood thesecationic counter ions can be exchanged to other ions through anion-exchange process.

Most preferably, the organic group is a substituted or unsubstitutedsulfophenyl group or a salt thereof; a substituted or unsubstituted(polysulfo)phenyl group or a salt thereof; a substituted orunsubstituted sulfonaphthyl group or a salt thereof; or a substituted orunsubstituted (polysulfo)naphthyl group or a salt thereof. A preferredsubstituted sulfophenyl group is hydroxysulfophenyl group or a saltthereof. Specific organic groups having an ionizable functional groupforming an anion are p-sulfophenyl, 4-hydroxy-3-sulfophenyl, and2-sulfoethyl. More preferred examples include p-C₆H₄SO₃ ⁻Na⁺ and C₆H₄CO₂⁻Na⁺.

Amines represent examples of ionizable functional groups that formcationic groups and can be attached to the same organic groups asdiscussed above for the ionizable groups which form anions. For example,amines may be protonated to form ammonium groups in acidic media.Preferably, an organic group having an amine substituent has a pKb ofless than 5. Quaternary ammonium groups (—NR₃ ⁺), quaternary phosphoniumgroups (—PR₃ ⁺), and sulfonium groups (—SR₂ ⁺) also represent examplesof cationic groups and can be attached to the same organic groups asdiscussed above for the ionizable groups which form anions. Preferably,the organic group contains an aromatic group such as a phenyl or anaphthyl group and a quaternary ammonium, a quaternary phosphoniumgroup, or a sulfonium group. The aromatic group is preferably directlyattached to the carbon product. Quaternized cyclic amines andquaternized aromatic amines can also be used as the organic group. Thus,N-substituted pyridinium compounds, such as N-methyl-pyridyl, can beused in this regard. Examples of organic groups include, but are notlimtied to, 3-C₅H₄N(C₂H₅)⁺, C₆H₄C₅H₅ ⁺, C₆H₄COCH₂N(CH₃)₃ ⁺,C₆H₄COCH₂(NC₅H₅)⁺, 3-C₅H₄N(CH₃)⁺, and C₆H₄CH₂N(CH₃)₃ ⁺. Counter ions tothose groups include, but are not limited to, Cl⁻, NO₃ ⁻, OH⁻, andCH₃COO⁻. It is understood that these anionic counter ions can beexchanged to other ions through an ion-exchange process.

As stated earlier, non-ionic hydrophilic groups can be used. Examples ofthe non-ionic hydrophilic groups include, but are not limited to, groupshaving no apparent ionic change and can not be transformed to have anionic charge, such as polymers/oligomers of the ethylene oxide,propylene oxide, other alkylene oxides, glycols, alcohols, and the like.

As part of the present invention, it is preferred that the amount ofhydrophilic organic groups attached to the carbon product is controlledin order to avoid making the modified carbon product overly hydrophilic.In particular, as a preferred embodiment of the preferred invention, thetreatment level, which is expressed in terms of μmol/m² of carbon, ofthe hydrophilic organic group on the carbon product is from about 0.04μmol/m² to about 6 μmol/m², more preferably from about 0.1 μmol/m² toabout 2 μmol/m², and most preferably from about 0.2 μmol/m² to about 0.8μmol/m².

In a more preferred embodiment of the present invention, the carbonproduct which has attached at least one hydrophilic organic group, alsohas attached at least one hydrophobic organic group as well to betterpromote a hydrophobic/hydrophilic balance in the active layer. Thehydrophobic organic groups can be the same as described above. Forpurposes of this preferred embodiment of the present invention, thetreatment level of the hydrophobic organic group on the modified carbonproduct is preferably from about 0.04 μmol/m² to about 6 μmol/m², morepreferably from about 0.1 μmol/m² to about 4 μmol/m², and mostpreferably from about 0.5 μmol/m² to about 3 μmol/m².

Alternatively, instead of a dual or multi-treated modified carbonproduct as described above in the preferred embodiment, two or moredifferent types of modified carbon products can be used, in particular,one modified carbon product can be a carbon product having attached atleast one hydrophilic organic group and a second type of modified carbonproduct can be used which is a carbon product having attached at leastone hydrophobic organic group. Then, in this embodiment, a mixture ofthe two different types of modified carbon products can be used to formthe active layer along with the presence of a binder.

Any carbon products that are used in air-diffusion electrodes can beused in the present invention. Examples of such carbon products include,but are not limited to, graphite, carbon black, vitreous carbon,activated charcoal, carbon fiber, activated carbon, and carbon aerogel.Catalyzed carbon products that are used in air-diffusion electrodes canalso be used in the present invention, wherein surface modification canbe performed either before or after the catalization step. Finelydivided forms of the above are preferred. Further, mixtures of differentcarbon products can be used. Preferably, the carbon product used iscapable of reacting with a diazonium salt to form the above-mentionedcarbon products. The carbon may be of the crystalline or amorphous type.In addition, mixtures of different types of modified carbon productswith or without unmodified carbon products can also be used in thepresent invention as one embodiment.

The organic groups which are attached onto the carbon product to form amodified carbon product can be attached by the methods described in thefollowing U.S. Patents and Publications which are all incorporated intheir entirety by reference herein: U.S. Pat. Nos. 5,851,280; 5,837,045;5,803,959; 5,672,198; 5,571,311; 5,630,868; 5,707,432; 5,803,959;5,554,739; 5,689,016; 5,713,988; WO 96/18688; WO 97/47697; and WO97/47699.

Besides the presence of the modified carbon product in one or morecomponents of the electrode described above, conventional ingredientsused in electrodes can also be present in the electrodes of the presentinvention. For instance, fluorine containing compounds typically used inair-diffusion electrodes can also be used in the present invention suchas polytetrafluoroethylene in the blocking layer. Likewise, in theactive layer, a perfluoric sulphonic acid polymer sold under the tradename Nafion® can be used with the modified carbon products. However, onepreferred advantage of the present invention is the ability to reducesuch fluorine containing compounds in the blocking layer and/or activelayer. The proper choice of organic groups attached onto the carbonproduct to form the modified carbon product can lead to a decrease ifnot an elimination of fluorine containing compounds which in the pasthave been used in conjunction with carbon black in order to promote thehydrophilic and/or hydrophobic properties discussed above. The reductionor elimination of such fluorine containing compounds can greatly reducethe cost of the electrodes and thus the present invention provides avery economical electrode. Preferably, for purposes of the presentinvention, the amount of the reduction of a hydrophobic fluorinecontaining compound in the blocking layer is from about 10 to about 100%by weight, more preferably from about 40 to about 100% by weight.Further, with respect to the active layer, preferably the amount ofreduction of the fluorine containing compound is from about 10 to about100% by weight, more preferably from about 60 to about 100% by weight.

The electrodes containing the modified carbon products of the presentinvention also permit the extension of the service life of theelectrode. Generally, at least two processes could negatively influencethe performance of the gas diffusion electrode during its long timeoperation as an oxygen reduction cathode: oxidation of the carbonsurface due to the decomposition of the hydroperoxide anion HO₂ ⁻ whichis an intermediate product of the oxygen reduction; and the excessivewetting of the internal microporous structure. It is well knownexperimentally that the second of the above processes predominates inthe electrode performance failure during long time operation.

Gas diffusion electrodes are fabricated so as to produce the maximumarea of a three-phase interface, that is the maximum area of contactbetween the electrolyte, the gaseous reactant and the catalyst supportedon carbon as the electronically conducting material. As discussedbefore, the partially wet-proofed catalyst layer is currently achievedby the mixture of the catalysed carbon particles with a hydrophilicagent. The additional use of PTFE was considered necessary so that thecatalyst would not flood from the presence of liquid electrolyte. Thenet result is a structure in which the PTFE selectively wets parts ofthe catalyst agglomerates on a rather random basis. The areas where thecarbon carrying the catalyst has become wetted by PTFE are hydrophobic,producing the gas pores, whereas those areas not covered by a PTFE filmbecome the hydrophilic electrolyte pores. The long-term stability of thethree-phase interface in hydrophobic porous Teflon-bonded carbonelectrodes is difficult to achieve. PTFE doesn't dissolve in any knownsolvent and consequently the conventional process of fabricating anelectrode is complicated by the use of a liquid suspension. Inparticular, when the electrodes are made in this fashion, it is quitedifficult to control the electrode structure and the porosity.

Moreover catalyst utilization has been rather poor because of the natureof the interface. It was found that, in conventional electrodes, a largepart of the catalyst is not effective. The electrochemical reactiontakes place only in those areas where catalyst is accessible both to thereactant gas and the electrolyte. PTFE makes the catalytic layer partlyimpermeable to the electrolyte so that the catalyst efficiency islowered, also resulting in the decrease of the electrode performance. Onthe other hand, a large amount of PTFE is required in the gas diffusionlayer to prevent the electrolyte diffusivity over a long period of time.This results in the reduction of the gas mass transport efficiency dueto the blocking effect of PTFE inside the fine porous structure.

Since the modified carbon products of the present invention promotehydrophobic and/or hydrophilic properties on a molecular scale, there isno random wetting of the carbon products and a very even distribution ofthe wetting characteristics exists throughout the active layer forinstance. Thus, the unwanted excessive wetting of the carbon productscan be avoided throughout the entire active layer which then leads to along term operation thus promoting the extension of the service life ofthe electrode. Further, with respect to the blocking layer, with amodified carbon product having attached hydrophobic organic groups, theblocking layer quite effectively blocks any electrolyte and permits thegreatest amount of air diffusion.

Besides air electrodes, the present invention relates to gas diffusionelectrodes in general, wherein the active layer and/or blocking layerthat may be present in gas-diffusion electrodes can include modifiedcarbon products as described above and serve the same function as themodified carbon products incorporated in the active layer and/orblocking layer of the electrode. Gas-diffusion electrodes, which includeair-diffusion electrodes, prepared with modified carbon material havebroad applications. One example of a gas diffusion electrode applicationwould be a phosphoric acid type fuel-cell using a pair of gas diffusionelectrodes. Such gas diffusion electrodes are described, for instance,in U.S. Pat. Nos. 5,846,670; 5,232,561; and 5,116,592, and allincorporated in their entirety by reference herein. Other applicationsare described in EP 435835, (Electro-plating); U.S. Pat. Nos. 5,783,325;5,561,000; 5,521,020 (Solid polymer electrolyte fuel cells); U.S. Pat.No. 5,733,430 (Sodium chloride electorlysis); U.S. Pat. No. 5,531,883(Ozone generation cells); U.S. Pat. No. 5,302,274 (Gas Sensor); U.S.Pat. No. 4,892,637 (Alkali chloride electrolyzers, air cells, and fuelcells); EP 327 018 A2 (Biosensors); A. Kaishera et al., Sens. Actuators,1995, 1327 ((1-3) (Biosensors), all are incorporated herein in theirentirety by reference.

The present invention can be used in a variety of gas diffusionelectrodes. For instance, but without limiting the present invention,the present invention can be used in large scale industrialapplications, such as chemical production. Examples of such industrialapplications include, but are not limited to, chloro-alkali production(e.g., the production of sodium hydroxide also known as salt splittingand chlorine production); hydrogen peroxide production; and the like.The present invention can also be used, as discussed above, in fuelcells, metal/air batteries, electro-platting (e.g., using hydrogen gas);ozone production, carbon dioxide decomposition; sensors for suchchemicals as ozone, oxygen, nitrogen dioxide, and the like; enzyme/gasdiffusion electrodes (e.g., biosensors); and the like. Each of theseapplications can incorporate the modified carbon material of the presentinvention in the electrode to obtain the benefits discussed above andone skilled in the art in view of the disclosure set forth in thispresent invention can apply this present invention to these variousapplications and therefore are considered part of the presentapplication.

The following examples further illustrate aspects of the presentinvention but do not limit the present invention.

EXAMPLES Example 1 (0.5 mmol/g Treatment with 3-Trifluoromethyl Aniline)

Preparation of a Hydrophobic Carbon Black Product with Diazonium Salt

This example illustrates the preparation of a hydrophobic carbon blackproduct of the present invention. A fluffy VXC-72R carbon black with asurface area of 254 m²/g and a DBPA of 192 cc/100 g was used. Fortygrams of the fluffy carbon black were added to a solution of 3.22 g of3-trifluoromethyl aniline dissolved in 900 g of de-ionized water and 1.8g of 70% nitric acid at 70° C. Then 100 mL of iso-propanol was added toassist the wetting of carbon black. To the reaction solution, 1.38 g ofsodium nitrite dissolved in 30 g of deionized water was added drop bydrop over a period of several minutes and stirred rapidly, to produce adiazonium salt, which reacted with the carbon black. The resultingreaction mixture was stirred rapidly for two more hours before cooledback to room temperature. Surface modified carbon black product was thenfiltered out by vacuum filtration. The crude modified carbon black wasdried at 70° C. overnight. The carbon black product contained 1.36% offluorine (equivalent to 0.24 mmol/g of surface 3-trifluoromethyl-phenylattachment to carbon black) after Soxhlet extraction with methanolovernight.

Example 2 (0.25 mmol/g Treatment with 3-Trifluoromethyl Aniline

Preparation of a Hydrophobic Carbon Black Product with Diazonium Salt

Modified carbon black product of this example was prepared following theprocedure described in Example 1; except, 1.61 g of 3-trifluoromethylaniline, 0.9 g of nitric acid, and 0.69 g of sodium nitrite were used.The carbon black product contained 0.68% of fluorine (equivalent to 0.12mmol/g of surface 3-trifluoromethyl-phenyl attachment to carbon black)after Soxhlet extraction with methanol overnight.

Example 3 (0.1 mmol/g Treatment with 3-Trifluoromethyl Aniline)

Preparation of a Hydrophobic Carbon Black Product with Diazonium Salt

Modified carbon black product of this example was prepared following theprocedure described in Example 1; except, 0.64 g of 3-trifluoromethylaniline, 0.36 g of nitric acid, and 0.28 g of sodium nitrite were used.The carbon black product contained 0.30% of fluorine (equivalent to 0.05mmol/g of surface 3-trifluoromethyl-phenyl attachment to carbon black)after Soxhlet extraction with methanol overnight.

Example 4 (0.1 mmol/g, Sulfanilic Acid)

Preparation of a Hydrophilic Carbon Black Product with Diazonium Salt

This example illustrates the preparation of a hydrophilic carbon blackproduct of the present invention. A fluffy VXC-72R carbon black with asurface area of 254 m²/g and a DBPA of 192 cc/100 g was used. Fortygrams of the fluffy carbon black were added to a solution of 0.70 g ofsulfanilic acid dissolved in 480 g of dc-ionized water at 70° C. To thatreaction solution, 0.28 g of sodium nitrite dissolved in 20 g ofde-ionized water was added drop by drop over a period of several minutesand stirred rapidly, to produce a diazonium salt, which reacted with thecarbon black. The resulting reaction mixture was stirred rapidly for twomore hours before cooled back to room temperature. The reaction mixturewas poured into a drying dish and dried at 70° C. for 2 days. The carbonblack product contained 0.8% of sulfur (There was 0.5% sulfur on thestarting VXC-72R, so that 0.3% of sulfur were introduced by the surfacemodification reaction which was equivalent to 0.09 mmol/g of surfaceattachment to carbon black) after Soxhlet extraction with tolueneovernight and then methanol overnight.

Example 5 (0.1 mmol/g of Sulfanilic Acid, 0.2 mmol/g 3-Tri-FluoromethylAniline)

Preparation of a Hydrophobic/Hydrophilic Carbon Black Product withDiazonium Salt Dual Treatment.

This example illustrates the preparation of a hydrophobic/hydrophiliccarbon black product of the present invention. A fluffy VXC-72R carbonblack with a surface area of 254 m²/g and a DBPA of 192 cc/100 g wasused. Forty grams of the fluffy carbon black were added to a solution of0.70 g of sulfanilic acid dissolved in 480 g of de-ionized water and 20g of iso-propanol at 70° C. To that reaction solution, 0.28 g of sodiumnitrite dissolved in 20 g of de-ionized water was added drop by dropover a period of several minutes and stirred rapidly, to produce adiazonium salt, which reacted with the carbon black. The resultingreaction mixture was stirred rapidly for two hours before an additional220 g of de-ionized water and 30 g of iso-propanol were added. Then, 1.3g of 3-trifluoromethyl aniline and 0.52 g of 70% nitric acid were added.To that reaction solution, 0.56 g of sodium nitrite dissolved in 20 g ofde-ionized water was added drop by drop over a period of several minutesand stirred rapidly, to produce a second diazonium salt, which reactedwith the carbon black. The resulting reaction mixture was stirredrapidly for two more hours before cooled back to room temperature. Thereaction mixture was poured into a drying dish and dried at 70° C. for 2days. The carbon black product contained 0.83% of sulfur (Mere was 0.5%sulfur on the starting VXC-72R, so that 0.33% of sulfur were introduceddby the surface modification reaction which was equivalent to 0.1 mmol/gof surface attachment to carbon black.), and 0.67% of fluorine(equivalent to 0.11 mmol/g of surface hydrophobic treatment to carbonblack) after Soxhlet extraction with toluene overnight and then methanolovernight.

Example 6 (PTFE, FEP Treatment)

Preparation of a Fluoro-Polymer Bonded Carbon Black Product

Twenty grams of carbon black material prepared in Example 1 were madeinto fluffy form by chopping the carbon black in an industrial blenderfor 3 minutes. Those carbon black were then added to a beaker with 730 gof de-ionized water and 40 mL of iso-propanol at 80° C. The carbon blacksuspension was stirred rapidly for 90 minutes before 5.55 g of DuPontTeflon 30 (polytetrafluoroethylene, PTFE water dispersion), and 17.92 gof DuPont Teflon 121A (fluorinated ethylene propylene co-polymerFEP-water dispersion) were added. An additional 300 g of de-ionizedwater was added and the mixture was stirred for additional 2 hours at80° C. After cooling the mixture to room temperature the fluoro polymerbonded carbon black product was isolated by vacuum filtration. Thecarbon material was then dried at 150° C. overnight followed by 310° C.overnight.

Example 7 (Example 2, with PTFE, FEP Treatment)

Preparation of a Fluoro-Polymer Bonded Carbon Black Product

Fluoro-polymer bonded carbon material of this example was preparedaccording to the procedure in Example 6, except the starting carbonblack material used was prepared in Example 2.

Example 8 (Example 3, with PTFE, FEP Treatment)

Preparation of a Fluoro-Polymer Bonded Carbon Black Product

Fluoro-polymer bonded carbon material of this example was preparedaccording to the procedure in Example 6, except the starting carbonblack material used was prepared in Example 3.

Example 9 (Example 1, with FEP Treatment)

Preparation of a Fluoro-Polymer Bonded Carbon Black Product

Fifteen grams of carbon black material prepared in Example 1 were madeinto fluffy form by chopping the product in an industrial blender for 3minutes. The carbon black were then added to a beaker with 550 g ofde-ionized water and 30 mL of iso-propanol at 80° C. The carbon blacksuspension was stirred rapidly for 60 minutes before 13.44 g of DuPontTeflon 121A were added. Additional 200 g of de-ionized water was addedand the mixture was stirred for additional 2 hours at 80° C. Aftercooling the mixture to room temperature the fluoro polymer bonded carbonblack product was isolated by vacuum filtration. The carbon material wasthen dried at 150° C. overnight and followed by 310° C. overnight.

Example 10 (Example 2, with FEP Treatment)

Preparation of a Fluoro-Polymer Bonded Carbon Black Product

Fluoro-polymer bonded carbon material of this example was preparedaccording to the procedure in Example 9, except the starting carbonblack material used was prepared in Example 2.

Example 11 (Example 3, with FEP Treatment)

Preparation of a Fluoro-Polymer Bonded Carbon Black Product

Fluoro-polymer bonded carbon material of this example was preparedaccording to the procedure in Example 9, except the starting carbonblack material used was prepared in Example 3.

Example 12

Preparation of an Active Layer Material

For the preparation of the active layer powder, the modified carbonproduct of Example 5 was combined with isopropyl alcohol to form amixture having a paste-like consistency.

Example 13

Preparation of an Active Layer Material

In this Example, the modified carbon product of Example 5 was mixed witha commercially available fluorinated ethylene propylene copolymer powder(FEP 5328000, Du Pont) in a ratio of about 2 part by weight carbon and 1part by weight FEP. The polymer serves as a binder for the carbon blackparticles to ensure the physical integrity of the electrode structureand its mechanical strength.

The above carbon /FEP mixture was mechanically blended and grinded forseveral minutes and then the resulting active layer powder was added toisopropanol to form a material having a paste-like consistency.

Example 14

Preparation of an Active Layer Material

In this example, the modified carbon product of Example 5 was made morehydrophobic by impregnation in a FEP solution. The modified carbonproduct was first stirred in distilled water using 50 mls of water pergram of carbon black.

A separate aqueous dispersion was prepared by diluting commerciallyavailable ethylene propylene co-polymer dispersion (FEP 120-N, Du Pont)with distilled water using 18 mls of water per ml of said dispersion.Then the diluted FEP emulsion was added to the modified carbon slurry tomake a mixture containing 2 part by weight carbon and 1 part by weightFEP. The mixture was stirred for 90 min at 85° C. and then filteredthrough a fine pore membrane. The resulting carbon/FEP paste was driedin an oven at 110° C. to remove solvent (water and isopropyl alcohol).Finally the mixture was dried at a temperature that ranged from 260 to310° C. for 24 h to remove the residual surfactant agents.

The product was then micronized to a fine powder and blended inisopropanol to obtain a dispersion to be used as active layer material.The amount used was 40 mls of solvent per g of modified carbon product.

Example 15

Preparation of a Catalyzed Active Layer Material

A somewhat different process was required for the preparation of acatalysed active layer. The catalytic material was a macrocycled chelatecompound, which was a cobalt porphyrin. The addition of the cobaltporphyrin to carbon was achieved by intimately mixing the modifiedcarbon product and catalyst, followed by heat treatment in furnace undera continuous flow of inert gas. The carbon-supported catalyst was heatedto high temperature (900° C. for 1 h) in order to be activated. Theelectrocatalytic activity of cobalt-containing macrocyclicssignificantly improved after heating the carbon-supported material tohigh temperatures.

After such pyrolisis, the pyrolyzed mixture was finely grounded. Fromthis point on the aforementioned carbon product was made hydrophilicfollowing the procedure described in Example 4 or, otherwise, it wasmade partially hydrophobic and partly hydrophilic following theprocedure described in Example 5. Finally the catalyzed active layerpowder was prepared beginning from the catalyzed and modified carbonparticles described above and carrying out the procedure of Example 12or 13 or 14.

Example 16

For comparison purposes, three separate double-layer air electrodes werefabricated:

-   -   i) Electrode A was prepared from carbon powder modified as        described above.

First, 6 g of active paste prepared as described in Example 13 werespread over a nickel mesh to integrate the current collector into acomposite structure supported on a separable filter paper. Thisstructure was then air dried. Subsequently, 8 g of blocking powder inthe alcoholic dispersion as described in Example 1 were filtered on theabove-prepared active layer which provided a freshly deposited diffusionlayer. The resulting double-layer electrode was pressed at 120° C. and30 tons to form a sheet ranging in thickness from 0.50 to 0.58 mm.Finally the dried electrode was sintered at a temperature between 280°C. and 300° C. at moderate pressure.

-   ii) Electrode B was produced by following the conventional    techniques of the state-of-the-art, specifically electrode B was    produced according to the standard procedure described in the U.S.    Pat. No. 5,441,823 (Electric Fuel Ltd) and U.S. Pat. No. 4,927,514    (Eltech Systems Co.), incorporated in their entirety herein by    reference. This electrode was prepared from untreated Vulcan XC-72R    carbon black.

The powder for use in preparing the active layer was prepared bydispersing 7.5 weight parts of carbon and 2.5 weight parts of Nafion®(5% solution) together with isopropanol. The mixture was evaporated todryness with constant stirring at 65° C. and then the solid was furtherdried under vacuum at 110° C., overnight. The product was finely groundand blended with FEP powder at a ratio of 2: 1 by weight. The powder foruse in preparing a diffusion layer was provided by dispersing carbon indistilled water at 85° C. by means of an overhead stirrer. To this therewere added dispersions of PTFE and FEP sufficient to provide a weightratio of PTFE to FEP to carbon of 1: 3: 6. The mixture was furtherblended by stirring and then filtered through a fine pore membrane. Themoist solid was dried overnight in an oven at 150° C. and then heattreated at 310° C. for 12 h. Finally the product was ground. Ahydrophilic active layer was then prepared by first mixing 6.0 g of thenafionized carbon/FEP fine powder with few drops of isopropanol alcoholand then depositing the resulting paste on a separable filter paper ontowhich a nickel mesh current collector was placed. A mixture of thefinely blocking material (8.0 g) was added to isopropanol andhomogenised in a blender. The resulting dispersion was then filtered onthe above-prepared active layer which provided a freshly depositedblocking layer having a mud-like consistency. The filter paper wasremoved and the resulting matrix was dried at 120° C. while beingcompressed. The dried matrix was sintered at 285° C. under a moderatepressure.

-   iii) Electrode C was prepared in the same manner as electrode B,    except for using Shawinigan black (Chevron Chemical Company) instead    of Vulcan XC-72R.

Electrochemical Measurements

The activity of electrodes for the reduction of oxygen was determinedusing a conventional half-cell arrangement.

The air electrode was located on one of the walls of the container andit was held between a holder aid a stainless steel end plate acting as acurrent collector. The test electrode was mounted having the activelayer faced towards the electrolyte. The reactant gas (air or oxygen)was fed at a constant flow rate of 12 nl/h from the rear. The reactantgas was introduced into the holder from a side hole and released from asecond hole. The electrolyte consisted of 7.5 M KOH and was stirredthroughout the experiment. The potential of the working electrode wasrecorded against an Hg/HgO, KOH reference electrode. All potentials arereported with regard to this electrode. A nickel plate served as counterelectrode.

Electrochemical measurements were taken using a programmable powersupply which was interfaced with a personal computer for programming,data acquisition, plotting, and analysis.

The electrodes were tested galvanostatically for the evaluation ofperformance comparisons. A constant current density of 200 mA/cm² wasapplied in the life test of the electrodes. After set periods ofoperation the tests were temporarily interrupted to obtain thesteady-state current/potential curves. The test was performed by passinga current between the air electrode and the counter electrode, andmeasuring the resulting voltage drop between the air electrode and thereference electrode. The current was increased stepwise every 60 sec andthe potential was logged in the steady state. All of the denotedpolarization curves contained IR resistance. The electrodes were kept incontact with KOH 7.5 M for 24 h to achieve a complete soakage.

FIG. 1 shows the potential-time curves under a current load of 200mA/cm² and 23° C. for the electrodes prepared.

The electrodes showed a potential change during the so-calledbreak-in-time. The load potential starts more negative and graduallyincreases to more positive levels which can be 0.1 V or more higher thanthe initial values when stabilised. The phenomenon was attributed to awetting process in the active layer where the electrolyte slowly reachesa maximum contact area with the catalytic sites.

It can be seen that the performance of the untreated carbon-basedelectrodes (Electrode B and C) deteriorated rapidly after a service offew hours. The decrease of activity can be explained by a reduction ofsurface area available at the electrochemical reaction. A progressiveflooding of the electrodes by the liquid electrolyte hindered theregular flow of reagent gases through the porous structure.

It was evident that the performance of the modified carbon-basedelectrode (Electrode A) was superior to those obtained with untreatedcarbon (Electrode B and C). The long term stability of Electrode A and Chave been tested. In FIG. 2 it can be seen that the degradation of theElectrode C was high during the first 100 h, after which the electrodebecame stable. Viceversa it is found that the Electrode A has so farbeen stable without appreciable deterioration for more than 500 h.

The polarisation curve of modified carbon-based electrode (Electrode A)which operated as oxygen reduction cathode is presented in FIG. 3. Thesame figure also presents the curves for the electrodes prepared withuntreated Shawinigan-based carbon (Electrode C). These polarizationcurves were recorded after 50 h of electrode operation in an oxygenreduction mode at 200 mA/cM². In earlier experiments, it was found thatthis procedure resulted in stabilization of the electrode performance.

The results show that the modified carbon-based electrode have betterperformance than conventional electrodes. The conventional electrodeshave a higher ohmic resistance which was indicated by the higher slopeof the polarization curves. The maximum activity of the modifiedcarbon-based electrode overpassed the one obtained with untreatedShawinigan carbon-based electrode. Polarisation curves in FIG. 3indicated that Electrode A can sustain a load current-density of 200mA/cm² with a polarisation of about −395 mV vs Hg/HgO. By contrast,higher polarisation values are observed for the Electrode B. A gain ofalmost 320 mV at 200 mA/cm² for the electrode with modified carbonproducts was observed. These results demonstrated the benefits ofpreparing carbon supports in which carbon is modified with functionalgroups on its surface.

Chemically treated carbon supports are much suitable from thehydrophobic point of view and a long life of the gas diffusionelectrodes can be secured without the deterioration of the performancedue to the permeation of the electrolyte into the gas feed channels.

Other embodiments of the present invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the invention disclosed herein. It is intended that the specificationand examples be considered as exemplary only, with a true scope andspirit of the invention being indicated by the following claims.

1. A metal-air battery comprising a gas diffusion electrode comprising ablocking layer and an active layer, wherein said blocking layer or saidactive layer, or both comprise at least one modified carbon product andat least one binder, said modified carbon product comprising at leastone carbon product having attached at least one organic group.
 2. A fuelcell comprising a gas diffusion electrode comprising a blocking layerand an active layer, wherein said blocking layer or said active layer,or both comprise at least one modified carbon product and at least onebinder, said modified carbon product comprising at least one carbonproduct having attached at least one organic group.
 3. The fuel cell ofclaim 2, wherein the fuel cell is a solid polymer electrolyte fuel cell.4. The fuel cell of claim 2, wherein the fuel cell is an alkali chloridefuel cell.
 5. A electroplating electrode comprising a gas diffusionelectrode comprising a blocking layer and an active layer, wherein saidblocking layer or said active layer, or both comprise at least onemodified carbon product and at least one binder, said modified carbonproduct comprising at least one carbon product having attached at leastone organic group.
 6. A sodium electrolyzer electrode or an alkalichloride electrolyzer comprising or a sodium chloride a gas diffusionelectrode comprising a blocking layer and an active layer, wherein saidblocking layer or said active layer, or both comprise at least onemodified carbon product and at least one binder, said modified carbonproduct comprising at least one carbon product having attached at leastone organic group.
 7. An ozone generator electrode comprising a gasdiffusion comprising a blocking layer and an active layer, wherein saidblocking layer or said active layer, or both comprise at least onemodified carbon product and at least one binder, said modified carbonproduct comprising at least one carbon product having attached at leastone organic group.
 8. A gas sensor comprising a gas diffusion electrodecomprising a blocking layer and an active layer, wherein said blockinglayer or said active layer, or both comprise at least one modifiedcarbon product and at least one binder, said modified carbon productcomprising at least one carbon product having attached at least oneorganic group.
 9. The gas sensor of claim 8, wherein the sensor is anozone sensor, an oxygen sensor or a nitrogen dioxide sensor.
 10. Analkali chloride air cell comprising a gas diffusion electrode comprisinga blocking layer and an active layer, wherein said blocking layer orsaid active layer, or both comprise at least one modified carbon productand at least one binder, said modified carbon product comprising atleast one carbon product having attached at least one organic group. 11.A biosensor comprising a gas diffusion electrode comprising a blockinglayer and an active layer, wherein said blocking layer on said activelayer, or both comprise at least one modified carbon product and atleast one binder, said modified carbon product comprising at least onecarbon product having attached at least one organic group.
 12. Afacility comprising a chloro-alkali production facility, a hydrogenperoxide production facility or a carbon dioxide decomposition facility,the facility comprising a gas diffusion electrode comprising a blockinglayer and an active layer, wherein said blocking layer or said activelayer, or both comprise at least one modified carbon product and atleast one binder, said modified carbon product comprising at least onecarbon product having attached at least one organic group.
 13. Anenzyme/gas diffusion electrode comprising a gas diffusion electrodecomprising a blocking layer and an active layer, wherein said blockinglayer or said active layer, or both comprise at least one modifiedcarbon product and at least one binder, said modified carbon productcomprising at least one carbon product having attached at least oneorganic group.